The use of radiation of various types and energies for charging, for machining workpieces to be machined, or for altering material properties of workpieces to be machined has become widespread in the art for a wide range of fields of application.
In this context, photon radiation (in other words in particular charging with light, charging with X-ray radiation, UV light, infra-red light and the like) is not the only possible type of radiation; in particular, particle radiation may also be considered. In this context, the particles may be substantially as desired (“particles” in this context meaning in particular particles which have a rest mass, even though it may be extremely small). Hadrons and leptons may be mentioned purely by way of example, in particular including neutrinos, electrons, positrons, pions, mesons, protons, neutrons, atomic nuclei (for example He nuclei), atoms or molecules and ions (in particular including heavy ions such as oxygen ions, helium ions, neon ions or carbon ions).
What all these types of radiation have in common is that the radiation deposits a particular energy in the item charged with radiation. However, in some cases the manner in which this energy is deposited varies greatly. Whilst for example in the case of photon radiation the energy loss is related approximately exponentially to the material penetrated over wide energy ranges, particle beams, in this case in particular hadron particles (especially protons, ions and heavy ions), have a pronounced Bragg peak. The particles thus initially lose comparatively little energy on the path thereof upon penetrating material. Shortly before the particles come to rest, the majority of the energy is released into the material charged with the radiation. As a result of this Bragg peak, not only two-dimensionally structured dose charges, but in particular also three-dimensionally structured dose charges can be realised (in other words different deposited radiation doses at different depths in the irradiated object).
Not only may the type of radiation used vary, but so also may the type of objects charged with radiation. To name just a few technical fields of application, possible examples relating to charging with protons in structuring processes include masks and material removal or material application in the manufacture of structured semiconductor components (such as memory elements, microprocessors and the like). Photons may also be used for cutting and/or welding workpieces (in particular if the photon radiation is in the form of a high-energy laser beam).
One example application for electron beams is electron beam welding, by means of which for example two metal workpieces can be welded together. Naturally, separation and structuring processes are also conceivable.
In medicine and veterinary medicine, radiation is used for therapeutic purposes. For example, it is known to use X-ray radiation for producing X-ray images (including three-dimensional images from CT (computed tomography) methods). Electron beams have also been used in medicine for several decades, for example for treating cancerous tumours. Treatment of tumours using protons and ions (in particular heavy ions) has also now become well established in medicine. Because of the previously described Bragg peaks of protons/ions/heavy ions, it is possible to charge a three-dimensionally defined and structured region (in particular a tumour) in a patient with radiation in a targeted manner by controlling a particle beam accordingly (for example as part of a scanning process), whilst the surrounding tissue is largely unaffected. Precisions in the millimeter range are now possible.
In scanning methods, a thin particle beam (often referred to as a pencil-thin particle beam) is conventionally used, and can be deflected laterally (x-y plane) using suitable deflection magnets and controlled in terms of penetration depth using suitable energy variation. By varying deflection and energy accordingly, it is possible to “approach” the various volume regions to be charged with a dose of the object to be irradiated. Irradiation generally follows a radiation treatment plan. In this context, a particular radiation pattern is computer-simulated (in other words a sequence with different x-y deflections of the particle beam and suitable particle energies of the particle beam) and the respectively resulting dose input into the body charged with the radiation is calculated as a function of location. This is because, although the deposited dose in the irradiated object is concentrated on the region of the Bragg peak, a particular dose is nevertheless deposited (in particular in regions lying close to the radiation point along the particle path). In the context of the radiation treatment plan, attempts are made to optimise the particle beam guidance in such a way that there is charging with a particular minimum dose within a region to be treated (usually referred to as the CTV (clinical target volume)) of the object. By contrast, surrounding material (tissue) should be exposed to as low a dose as possible.
Particular problems occur if (sub-regions of) the object to be irradiated move. In this context, movement may include not only translational movements, but also twisting movements and/or compression or extension movements. In particular in combination with scanning methods, the movements of the object and those of the particle beam may “interfere” with one another and lead to comparatively poor radiation results if suitable countermeasures are not taken.
A method which has now become widespread so as to be able to irradiate moving target regions involves tracking the particle beam. In this context, the particle beam is readjusted in such a way that it compensates the movement of the target volume region in the object. With beam tracking of this type, it is in fact possible for the matrix dot which is actually to be irradiated (or the target radiation position and/or the target volume region) to be controlled substantially with the planned dose. However, because the movements in the object to be irradiated cannot be predicted during planning (in particular in combination with the movements of the particle beam), dose inputs which cannot be planned in advance occur in regions which do not correspond to the matrix point currently being irradiated. As a result of the accumulation of doses introduced outside the currently controlled matrix dot in the material, the doses introduced during an object machining process (or a therapy session) can lead to a very significant difference between the target plan and the dose distribution actually introduced.
It is therefore desirable to measure the movement of the object during the irradiation of said object, and, by using these measurements, to calculate what the actual dose distribution in the irradiated object is. Knowledge of this type can also be used for example within the object machining process (for example in that the radiation yet to follow is adapted accordingly), or else at a later time, in particular if the entirety of the machining is carried out in a plurality of object machining sessions separated in time.
Methods by which dose input monitoring of this type can be carried out have been proposed.
For example, German Offenlegungsschrift DE 10 2009 055 902 A1 has proposed a method in which a compensation value for the ith matrix position is calculated as a function of the determined dose which the ith matrix position has already received when irradiating the previous matrix positions, and a compensated particle flow for the ith matrix position is calculated as a function of the compensation value for the ith matrix position and of the nominal particle flow for the ith matrix position, so as to irradiate the ith matrix position with the compensated particle flow determined for the ith matrix position. The difference between the target dose and the actual dose in the ith matrix position is determined using a pre-calculated database for dose compensation, in matrix form, having a large number of individual field elements Dmik. The dose compensation proposed therein delivers fully usable results. However, experience has shown that application in practice is limited to very small volumes where the number of matrix positions is relatively small. For larger volumes (CTV sizes above approximately 10 cm3), however, the storage requirement for storing the field elements increases excessively. Specifically, the storage requirement for the field elements increases proportionally to the square of the number of matrix positions. Thus, volumes above approximately 10 cm3 result in storage requirements in the gigabyte range, and even with modern computers this can only be implemented with difficulty. Further, the time required for advance calculation of the field elements Dmik increases disproportionately.